Republican Paul Ryan performed pretty well. He made no major mistakes, and a CNN instant poll of viewers had him winning narrowly, 48 percent to 44 percent. But my assessment of the debate agreed with that of a CBS instant poll of uncommitted voters, who saw Biden as the landslide winner, 50 percent to 31 percent.

Don’t just consider the two surveys and call it a draw. If you ask whose prospects of winning the election were boosted Thursday night, President Obama’s or Mitt Romney’s, the answer is really not debatable. This was a moment the Obama campaign needed badly.

The only reason for such urgency, of course, was the sudden disappearance of Obama’s lead in the polls following his awful performance against Romney last week. If the real Obama had shown up in Denver, and not an ill-prepared impostor, the focus here would have been on Ryan: Could he pass the only-a-heartbeat-away test that has given some vice presidential candidates, notably Sarah Palin, such trouble? Instead, this debate was mostly about Biden: Could he stem the Romney-Ryan momentum and dispel the gloom that had settled over the Democratic Party?

Biden succeeded, as evidenced by the fact that the lion’s share of post-debate commentary was All About Joe. Critics said he smiled too much, interrupted his opponent too often, came across as too “hot” for a medium that is all about cool. But these complaints only reinforced the fact that Biden was the protagonist of the evening.

I thought Ryan’s best moment came at the end, when he gave his closing remarks; they were cogent, polished and well-delivered. It was the only time Biden allowed him to get up a head of steam, and he took advantage.

The worst moment for Ryan came when they were talking about unemployment. Moderator Martha Raddatz asked how and when the jobless rate could be brought below 6 percent. Both men ignored the question. Ryan gave a general answer about economic policy. Biden went on the attack, reminding viewers that Ryan had sent him two letters requesting funds from the 2009 stimulus package for businesses in his home state of Wisconsin — and said the money was needed to create jobs.

According to Republican dogma, the stimulus is supposed to have been a gargantuan waste of money that utterly failed to create jobs. That’s what Ryan preached — but not what he practiced. Until Raddatz changed the subject, Ryan looked pretty grim.

Later, during a long exchange about tax policy, Ryan mentioned the Kennedy administration and Biden pounced: “Oh, now you’re Jack Kennedy?” It was a perfect echo of Lloyd Bentsen’s famous put-down of Dan Quayle during their 1988 debate, and Ryan looked at Biden as if to say, “All right, that was a good one.”

Mostly, Biden did the obvious things that Obama failed to do. The debate started at 9 p.m.; at 9:24, he made the first of three references to Romney’s “47 percent” speech, in which the Republican nominee described nearly half of Americans as hopelessly dependent on government and unwilling to “take personal responsibility and care for their lives.” Later mentions came at 10:22 and in Biden’s closing remarks at 10:30.

Biden pressed Ryan on his plan to turn Medicare into a voucher program. He pressed on Romney’s failure to specify what tax deductions and loopholes he plans to eliminate, in order to replace the revenue that would be lost through cutting income tax rates by 20 percent. Would the deduction for mortgage interest have to go, or perhaps the deduction for health-care expenses? Ryan resolutely — and embarrassingly — refused to answer.

Biden challenged Ryan on all his tough talk about foreign policy and the Obama administration’s alleged weakness. What would he and Romney do differently about Syria? What would they do differently about Iran? About Afghanistan? Ryan tried to bob and weave, but he came close to acknowledging the truth: Romney would do essentially what Obama is doing.

As I said, these are pretty obvious lines of attack. Biden didn’t do anything fancy. But I think his performance will be enough to snap Democrats out of their funk — for now, at least.

At this debate, the real Biden showed up. At the next, on Tuesday at Hofstra University on Long Island, the real Obama had better do the same.

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Eugene Robinson writes a twice-a-week column on politics and culture, contributes to the PostPartisan blog, and hosts a weekly online chat with readers. In a three-decade career at The Post, Robinson has been city hall reporter, city editor, foreign correspondent in Buenos Aires and London, foreign editor, and assistant managing editor in charge of the paper’s Style section.